Introduction to "robots.txt"

There is a hidden, relentless force that permeates the web and its billions
of web pages and files, unbeknownst to the majority of us sentient beings. I'm
talking about search engine crawlers and robots here. Every day hundreds of them
go out and scour the web, whether it's Google trying to index the entire web, or
a spam bot collecting any email address it could find for less than honorable
intentions. As site owners, what little control we have over what robots are
allowed to do when they visit our sites exist in a magical little file called
"robots.txt."

"Robots.txt" is a regular text file that through its name, has special
meaning to the majority of "honorable" robots on the web. By defining a few
rules in this text file, you can instruct robots to not crawl and index
certain files, directories within your site, or at all. For example, you may
not want Google to crawl the /images directory of your site, as it's both
meaningless to you and a waste of your site's bandwidth. "Robots.txt" lets
you tell Google just that.

Creating
your "robots.txt" file

So lets get moving. Create a regular text file called "robots.txt", and make
sure it's named exactly that. This file must be uploaded to the root
accessible directory of your site, not a subdirectory (ie:
http://www.mysite.com but NOT
http://www.mysite.com/stuff/). It is
only by following the above two rules will search engines interpret the
instructions contained in the file. Deviate from this, and "robots.txt" becomes
nothing more than a regular text file, like Cinderella after midnight.

Now that you know what to name your text file and where to upload it, you
need to learn what to actually put in it to send commands off to search
engines that follow this protocol (formally the "Robots Exclusion
Protocol"). The format is simple enough for most intents and purposes: a
USERAGENT line to identify the crawler in question followed by one or
more DISALLOW: lines to disallow it from crawling certain parts of
your site.

1) Here's a basic "robots.txt":

User-agent: *
Disallow: /

With the above declared, all robots (indicated by "*") are instructed to
not index any of your pages (indicated by "/"). Most likely not what you
want, but you get the idea.

2) Lets get a little more discriminatory now. While every
webmaster loves Google, you may not want Google's Image bot crawling your
site's images and making them searchable
online, if just to save bandwidth. The below declaration will do the
trick:

User-agent: Googlebot-Image
Disallow: /

3) The following disallows all search engines and robots from
crawling select directories and pages:

This is interesting- here we declare that crawlers in general should not
crawl any parts of our site, EXCEPT for Google, which is allowed to
crawl the entire site apart from /cgi-bin/ and /privatedir/.
So the rules of specificity apply, not inheritance.

5) There is a way to use Disallow: to essentially turn it into
"Allow all", and that is by not entering a value after the semicolon(:):

User-agent: *
Disallow: /
User-agent: ia_archiver
Disallow:

Here I'm saying all crawlers should be prohibited from crawling our site,
except for Alexa,
which is allowed.

6) Finally, some crawlers now support an additional field called
"Allow:", most notably, Google. As its name implies, "Allow:" lets you
explicitly dictate what files/folders can be crawled. However, this field is
currently not part of the "robots.txt" protocol, so my recommendation is to
use it only if absolutely needed, as it might confuse some less intelligent
crawlers.

Per Google's FAQs for
webmasters, the below is the preferred way to disallow all crawlers from
your site EXCEPT Google: